1. Field of the Invention
The present invention relates to a latent Lewis acid catalyst suitable for use in a storage-stable, one-part, curable resin system.
2. Description of the Prior Art
Storage-stable, one-part, curable resin systems generally comprise two or more reactive or reaction-producing components stored in an intimately admixed, unreactive state which, ideally, react rapidly when subjected to a suitable stimulus, such as application of heat or mechanical shear.
Newell, U.S. Pat. No. 4,225,460, discloses latent Lewis acid catalysts for cationically polymerizable resin systems comprising microcapsules of a Lewis acid-polyhydric alcohol (preferably glycerol) complex encapsulated within shell walls of a crosslinked interfacial polycondensation product of an aromatic polyisocyanate, glycerol and a cycloaliphatic epoxide, a process for the preparation of the microcapsules, and latent Lewis acid concentrates comprised of the microcapsules dispersed in a liquid medium.
The concentrates of the Newell patent were defined as slurries of
A. rupturable, impermeable microcapsules ranging in size from about 0.1 to 400 micrometers having
(1) shell walls of a crosslinked interfacial polyurethane-polyether reaction product of an aromatic polyisocyanate having an equivalent weight of up to about 380 and a functionality of from about 2 to 6, glycerol and a cycloaliphatic polyepoxide having an equivalent weight of from about 70 to 220 and a functionality of from about 2 to 3, and PA0 (2) liquid fills comprising a Lewis acid-glycerol complex; and
B. A liquid medium selected from aromatic polyisocyanates as defined above and esters of an aromatic carboxylic acid and an alkyl, aralkyl or aryl alcohol, the ester containing up to about 40 carbon atoms.
The latent, one-part epoxy formulations made utilizing the catalysts of Newell, U.S. Pat. No. 4,225,460 have excellent shelf stability at room temperature. However, the capsule concentrates alone do not exhibit an adequate level of storage stability. Because of the reactivity of the isocyanates present in these capsule concentrates, the viscosities of the concentrates increase fairly rapidly at room temperature until they become unusable, usually within a period of one or two months. Refrigeration of the concentrates improves shelf-life, but results in difficulty in shipping and expense in storing.
It would be preferable to ship capsule concentrates separately from the curable epoxy resin in order to reduce shipping and storage costs, if the capsules concentrates remained stable during shipping and storage.